Atomic Structure & the Periodic Table

Atomic Structure & the Periodic Table

1. Mendeleev

   • Mendeleev's table is known as the periodic table. To create his table he examined the masses of the known elements and their properties. He then arranged the elements by mass, but he left gaps, where elements with similar properties might go and lined elements up with similar properties, rather than just with similar masses.
     • He organised elements with similar properties in rows originally, but then put them into columns, like they are today. Mendeleev also used the gaps he had left to make predictions. One example is eka-aluminium, which had very similar properties to gallium. All elements on the left of the table are metals, and all on the right are non-metals.

2. Structure of Atoms

   • Atoms are the smallest particles of an element which can take part in a reaction. They are too small to see directly, however after completing many experiments, we know that atoms have an even smaller nucleus and are made up from subatomic particles.
     • Neutrons- Have no charge, makes up part of the mass number
     • Electrons- Have a negative charge and are arranged in shells. Equal to the number of protons.
     • Protons- Positive charge and make up the other half of the mass number and are the atomic number. There are always the same number of protons in he same element

3. Modern Periodic Table

   • The atomic number is the number of protons in an element (the top number), the bottom number, the mass number, is the number of protons and the number of neutrons.
     • The RAM (Ar), is the mass of an element- its protons and neutrons added together. RFM, is the mass of multiple elements.
     • The elements in the modern periodic table are arranged in order of increasing atomic number rather than atomic mass. The horizontal rows are called periods and the columns are called groups. There are columns 1-7 and then group 0.
   • Isotopes are the same atom with a different number of neutrons. The presence of isotopes means that not all atomic numbers are whole. So, to work out the RAM, you multiply the RAM of on isotope by the proportion of it, and so on, and divide by 100.

4. Electron Shells

   • Electrons are arranged in shells around the nucleus. The way the electrons are arranged is called the electron configuration. The first shell can only contain 2 electrons and the second and third shell contain eight electrons, but the thrid shell can contain more. The electron configuration affects the position of an element in the periodic table. The number of electrons in the outer shell, is the group number of an element, except for the noble gases.